This disclosure relates to a device for assisting the user of a map in determining the proper compass bearing along a course selected between two points on the map.
Navigational plotters as used with charts and maps for air or sea navigation typically comprise a circular protractor and a pivoted linear scale. When the protractor is placed over a compass rose, the bearing to or from the center of the compass rose can be read directly along the center axis of the linear scale. However, determining a bearing along a course not intersecting a compass rose on the map requires additional manipulation and calculation of angular deviations. This presents substantial opportunities for human error.
A common device for determining the bearing along a course line plotted on a chart or map is a parallel ruler. This device is moved along the map in a progression until one edge is aligned with the compass rose on the chart or map and the bearing can then be read from the compass rose. This is time-consuming and is an indirect method.
An example of a conventional type of protractor-ruler combination used for navigational purposes is illustrated in U.S. Pat. No. 932,907, granted to Rydman in 1909. It comprises a circular protractor and elongated scale which can be used singly or in combination to determine compass headings and distances on a map. It is normally aligned with lines on the map or chart indicative of true north, and requires a secondary corrective disk for compass readings corrected for magnetic deviation. The manipulation of two protractor disks introduces a significant chance of error and confusion in navigation calculations. More recent patents having similar structures and presenting similar problems are those granted to Price; U.S. Pat. Nos. 3,813,783 and 3,855,706.
Another instrument for assisting in determining a true course heading is illustrated by the Sena U.S. Pat. No. 3,535,783. However, it is used in relation to reference lines on the map, and can directly read magnetic compass bearings only with respect to courses intersecting a map compass rose. A similar limitation arises when using the adjustable plotter shown in U.S. Pat. No. 3,280,464 to Warner, granted in 1966.
In contrast to these prior protractor-ruler combinations, the present device is capable of conventional use for determining distances and bearings, but further enables the user to directly read magnetic bearings corresponding to any straight line course on a map within the vicinity of a map compass rose. Such compass roses occur with regularity on navigational maps. In those distributed for air navigational use, a compass rose is imprinted about navigational radio signal station locations. The device disclosed herein permits the navigator to directly read the necessary magnetic compass bearing from one point on the map to another without any mental calculations or directions and without moving any elements across the map or chart.
It is an object of this invention to provide a physically simple device for permitting direct reading of a compass bearing along any selected course adjacent to a compass rose imprinted on a map or chart.
Another object of this invention is to provide a device which eliminates the need for calculations or corrections in determining magnetic bearings and which visually indicates the correct direction of travel by arrows corresponding to the direction of travel.
These and further objects will be evident from the following disclosure and the accompanying drawings.